Over the years I have tested many different time slots, and the best times are those based around normal meal times.

The two time slots I offer at weekends are:

11:30-1:30pm for lunchtime parties (great for 3/4 year olds)

3:30-5:30pm for tea time parties.

Weekdays can be more flexible, and I normally suggest 4:30-6:30pm so the children still get to eat at their normal meal time.